Magnetic disks and disk drives are conventionally employed for storing data in magnetizable form. Preferably, one or more disks are rotated on a central axis in combination with data transducing heads positioned in close proximity to the recording surfaces of the disks and moved generally radially with respect thereto. Magnetic disks are usually housed in a magnetic disk unit in a stationary state with a magnetic head having a specific load elastically in contact with and pressed against the surface of the disk. Data are written onto and read from a rapidly rotating recording disk by means of a magnetic head transducer assembly that flies closely over the surface of the disk. Preferably, each face of each disk will have its own independent head.
Disc drives at their most basic level work on the same mechanical principles as media such as compact discs or even records, however, magnetic disc drives can write and read information much more quickly than compact discs (or records for that matter!). The specific data is placed on a rotating platter and information is then read or written via a head that moves across the platter as it spins. Records do this in an analog fashion where the disc's grooves pick up various vibrations that then translate to audio signals, and compact discs use a laser to pick up and write information optically.
In a magnetic disc drive, however, digital information (expressed as combinations of “0's” and “1's”) is written on tiny magnetic bits (which themselves are made up of many even smaller grains). When a bit is written, a magnetic field produced by the disc drive's head orients the bit's magnetization in a particular direction, corresponding to either a 0 or 1. The magnetism in the head in essence “flips” the magnetization in the bit between two stable orientations. In currently produced hard disc drives, longitudinal recording is used. In longitudinal recording, the magnetization in the bits is flipped between lying parallel and anti-parallel to the direction in which the head is moving relative to the disc.
Newer longitudinal recording methods could allow beyond 100 gigabits per square inch in density. A great challenge however is maintaining a strong signal-to-noise ratio for the bits recorded on the media. When the bit size is reduced, the signal-to-noise ratio is decreased, making the bits more difficult to detect, as well as more difficult to keep stable.
Perpendicular recording could enable one to record bits at a higher density than longitudinal recording, because it can produce higher magnetic fields in the recording medium. In perpendicular recording, the magnetization of the disc, instead of lying in the disc's plane as it does in longitudinal recording, stands on end perpendicular to the plane of the disc. The bits are then represented as regions of upward or downward directed magnetization (corresponding to the 1's and 0's of the digital data).
Increasing areal densities within disc drives is no small task. For the past few years, technologists have been increasing areal densities in longitudinal recording at a rate in excess of 100% per year. But it is becoming more challenging to increase areal densities, and this rate is expected to eventually slow until new magnetic recording methods are developed.
To continue pushing areal densities in recording and increase overall storage capacity, the data bits must be made smaller and put closer together. However, there are limits to how small the bits may be made. If the bit becomes too small, the magnetic energy holding the bit in place may become so small that thermal energy may cause it to demagnetize over time. This phenomenon is known as superparamagnetism. To avoid superparamagnetic effects, disc media manufacturers have been increasing the coercivity (the “field” required to write a bit) of the disc.
In magnetic disk, “servo sectors” are pre-written to define data tracks. Traditionally, servo-sectors were written by a tool called servo-track writer. There is also a method to write servo sectors by means of magnetic-contact printing, to which the RAIL invention is related to. In magnetic disk media, there is a sector called “servo-sector” where the disk manufacturer prints data for the operation of the disk using a master by an imaging process. The servo-sector typically occupies about 5-10% of the disk capacity. As the areal density increases it is also desirable to decrease the size of the servo-sector. This decrease in the size of the servo-sector could be brought about by increases in the imaging process, known as patterning, for making the master.
Patterning is an operation that removes specific portions on the surface of the master. Photolithography is one of the terms used to identify the operation of patterning. Other terms used are photomasking, masking, microlithography and interference lithography.
Patterning is one of the important operations in disk media manufacturing. The goal of the operation is twofold. First, is to create in and on the master surface a pattern whose dimensions are as close to as the resolution of the images on the master. The pattern dimensions are referred to as the feature sizes or image sizes of the pattern. The second goal is the correct placement (called alignment or registration) of the pattern on the master. The entire pattern must be correctly placed on the master and the individual parts of the pattern must be in the correct positions relative to each other.
Lithography is a pattern transfer process similar to photography and stenciling. In the field of mastering the servo-patterned media (SPM), laser-beam and electron beam lithography are mature technologies. The system consists of an electron source that produces a small-diameter spot and a “blanker” capable of turning the beam on and off. The exposure must take place in a vacuum to prevent air molecules from interfering with the electron beam. The beam passes through electrostatic plates capable of directing (or steering) the beam in the x-.y direction on the SPM. This system is functionally similar to the beam steering mechanisms of a television set. Precise direction of the beam requires that the beam travel in a vacuum chamber in which there is the electron beam source, support mechanisms, and the substrate being exposed. Since the pattern required generates from the computer, there is no mask. The beam is directed to specific positions on the surface by the deflection subsystem and the beam turned on where a photoresist (also called a resist) is to be exposed. Larger substrates are mounted on an x-y stage and are moved under the beam to achieve full surface exposure. This alignment and exposure technique is called direct writing.
The pattern is exposed in the resist by either raster or vector scanning. Raster scanning is the movement of the electron beam side-to-side and down the wafer. The computer directs the movement and activates the blanker in the regions where the resist is to be exposed. One drawback to raster scanning is the time required for the beam to scan, since it must travel over the entire surface. In vector scanning, the beam is moved directly to the regions that have to be exposed. At each location, small square or rectangular shaped areas are exposed, building up the desired shape of the exposed area.
However, with the x-y stage-based lithography tools, accurate r-θ position control was difficult which led to the development of the electron-beam recorder with a rotating stage and a linear controller, which provided with an accurate r-θ position control. The r-θ position controlled lithography also made it possible to define small features, determined mainly by the beam-spot size, which is important for making a master for SPM as the density increases. One requirement for the SPM master is an accurate trackpitch control. It is affected by factors such as vibration, random beam deflection due to disturbances, precision and stability of linear actuator control. Despite extensive engineering efforts, it is difficult to achieve trackpitch variation (3σ) less than 10 nm, which is required for the SPM master for hard disks at 200 kTPI (tracks per inch). The limitation is mainly due to inability to control the beam position relative to the wafer (substrate) accurately, affected by the factors mentioned above. Another disadvantage of the e-beam lithography is the slow throughput.
On the other hand, there exists a technology called interferometric lithography. It can make fine periodic patterns, but it could not make patterns required for the SPM master, consisting of synchronous fields, position-error-signal (PES) bursts, and so on, in an arc shape representing head movement with a rotary actuator in a hard-disk drive.